Cardiac dysrhythmias are commonly known as irregular heart beats or racing heart. Two such heart rhythm irregularities are the Wolff-Parkinson-White syndrome and AV nodal reentrant tachycardia. These conditions are caused by an extraneous strand of muscle fiber in the heart that provides an abnormal short-circuit pathway for electric impulses normally existing in the heart. For example, in one type of Wolff-Parkinson-White syndrome, the accessory pathway causes the electric impulses that normally travel from the upper to the lower chamber of the heart to be fed back to the upper chamber. Another common type of cardiac dysrhythmia is ventricular tachycardia (VT), which may be a complication of a heart attack or reduction of blood supply to an area of heart muscle. This latter type of cardiac dysrhythmia is a life-threatening arrhythmia.
Atrial fibrillation (AF) is the most commonly occurring type of arrhythmia. It is associated with increased morbidity and mortality due to a higher incidence of thromboembolic events and hemodynamic compromise. In patients with disabling drug resistant AF, the ventricular response can be controlled by catheter ablation or modification of the atrioventricular (AV) nodal region, but this procedure is palliative since AF and its related risks are persistent. Pacemakers may be used to prevent recurrence of paroxysmal AF by either preventing the sinus bradycardia that triggers AF or reducing the interatrial conduction delay.
Non-surgical procedures such as management with drugs are favored in the treatment of cardiac dysrhythmias. However, some arrhythmias are not treatable with drugs, for example, disabling drug resistant AF, and have previously required surgery. According to these procedures, various incisions are made in the heart to block conduction pathways and thus divide the atrial area available for multiple wavelet reentry and abolish the arrhythmia. Alternatively, an Automatic Implantable Cardioverter/Defibrillator (AICD) can be surgically implanted into the patient, as described in U.S. Pat. No. 4,817,608 to Shapland et al. While these surgical procedures can be curative, they are associated with increased morbidity and mortality rates, and are extremely expensive. Even the use of an AICD requires major surgical intervention. However, patients of advanced age or illness, for example, cannot tolerate invasive surgery to excise the tachycardia focus which causes dysrhythmias.
Non-surgical, minimally invasive techniques have been developed which are used to locate cardiac regions responsible for tachycardia, and also to disable the short-circuit function of these areas. According to these techniques, electrical energy shocks are applied to the endomyocardium to ablate cardiac tissue in the arrhythmogenic regions and produce scars which interrupt the reentrant conduction pathways. The regions to be ablated are usually first determined by endocardiac mapping techniques. Mapping typically involves percutaneously introducing an electrode catheter into the patient, passing the electrode catheter through a blood vessel (e.g. the femoral vein or aorta) and into an endocardiac site (e.g., the atrium or ventricle of the heart), and inducing a tachycardia so that a continuous, simultaneous recording can be made with a multichannel recorder at each of several different endocardiac positions. When a tachycardial focus is located, as indicated in the electrocardiogram recording, it is marked by means of a fluoroscopic image so that cardiac arrhythmias at the located site can be ablated. A conventional electrode catheter provides electrical energy shocks to the tissue adjacent the electrode to create a lesion in the tissue. One or more suitably positioned lesions will create a region of necrotic tissue which will disable the malfunction caused by the tachycardial focus.
Conventional catheter ablation techniques have used catheters each having a single electrode fitted at its tip as one electrical pole. The other electrical pole is conventionally provided by a backplate in contact with a patient's external body part to form a capacitive coupling of the ablation energy source (DC, laser, RF, etc.). Other ablation catheters are known in which multiple electrodes are provided, such as the catheters disclosed in U.S. Pat. Nos. 5,239,999 to Imran, and 4,940,064 and 5,383,917, both to Desai.
Ablation is effected by applying energy to the catheter electrodes once the electrodes are in contact with the cardiac tissue. The energy can be, for example, RF, DC, ultrasound, microwave, or laser radiation. As between RF and DC ablation techniques, RF ablation is preferable because it does not require that the patient be anesthetized, and it produces more circumscribed and discrete lesions. Further, it avoids injury that may be caused by high voltages, for example, DC shock. When RF energy is delivered between the distal tip of a standard electrode catheter and a backplate, there is a localized RF heating effect. This creates a well-defined, discrete lesion slightly larger than the tip electrode.
The small size of the lesions produced by RF ablation has been perceived as one of the limitations of this technique. Unless the electrode has a large contact region, the lesion created by the simple RF technique delivered through a standard electrode catheter may not be large enough to ablate enough tissue to block ventricular tachycardia, for example, because the electrode tip area is usually only about 0.3 mm.sup.2 and the exit site of VT is typically only locatable to within 4-8 cm.sup.2 of the earliest site recorded by the endocardiac mapping catheter. Also, attempts have been made to provide an elongated electrode six, eight, ten or even twelve millimeters in length to cause longer lesions, and to allow more power to be delivered to the tissue. However, flexibility of the tips of such catheters is reduced, and a stiffer catheter tip increases the risk of myocardial wall perforation, which in turn increases the morbidity rate of ablation procedures using such catheters.
Several other techniques have been implemented to produce larger and deeper lesions, including the use of different energy sources such as ultrasound, microwave, and laser. Other methods include using a saline-perfused catheter tip to cool the electrode/tissue interface, allowing more power to be delivered.
One particular approach to increasing the size of the lesion is disclosed in U.S. Pat. No. 4,940,064 to Desai. A retractable array of four orthogonal electrodes surrounds a central tip electrode and is powered by a conventional RF power source. This array was found to produce an unsatisfactory lesion pattern (in the form of a plus "+" sign) because substantial areas between the electrodes remained unablated, and the increase of power to the electrodes only resulted in charring of the tissues and early fouling of the electrodes by coagulum formation. As a solution, Desai et al. proposed in U.S. Pat. No. 5,383,917 the use of a multi-phase power source to electrically drive the peripheral electrodes out of phase with respect to adjacent electrodes to create an electric potential between adjacent peripheral electrodes and thereby cause ablation in the regions between those adjacent electrodes (that is, lesions which connect the tips of the plus "+" pattern to one another). This solution dispenses with the use of an external return or passive electrode because ground potential is provided at the central tip electrode, but poses constraints on the dimensions of the electrodes for satisfactory operation, and does not produce a continuous, linear lesion among or between the electrodes. See col. 8, lines 19-30 of U.S. Pat. No. 5,383,917.
In another approach, a series of electrodes were arranged along a catheter shaft to demonstrate the feasibility of catheter ablation of typical human AF by the sequential application of radiofrequency energy in a system using a backplate. Haissaguerre, et al., "Successful Catheter Ablation of Atrial Fibrillation, " J. Cardiovascular Electrophysiology, 1994, Vol. 12, No. 5:1045-1052. While the investigators in this study cured the AF in the patient, they were unable to confirm whether the lesions produced at the site of each electrode joined together to form a continuous lesion.
What is needed in the art and has heretofore not been available is a power supply arrangement for independently and controllably driving a multiplicity of electrodes spaced along the distal end of a cardiac ablation catheter. Also needed in the art is an ablation system which incorporates such a power supply arrangement with an ablation catheter so that continuous, linear ablation lesions of a predetermined contour can be formed in the endomyocardium.